Highly Regioselective Friedländer Reaction

Synthesis of novel N-functionalized 4-aryl-tetrahydrobiquinoline-2,5-(1H,3H)-diones via one-pot three-component reaction: a joint experimental and computational study

A novel series of N-functionalized 4-aryl-tetrahydrobiquinoline-2,5-(1H,3H)-diones were synthesized in high yields by a one-pot three-component reaction involving 2-chloroquinoline-3-carbaldehydes, Meldrum’s acid, and enaminones (dimedone-based enaminones) in the presence of K2CO3 in CH3CN under reflux condition. To gain a deep insight on the mechanism of the reaction, an extensive series of quantum mechanics calculations in the framework of density functional theory (DFT) were carried out for supporting the suggested reaction pathway.

Protic ionic liquids as catalysts for a three-component coupling/hydroarylation/dehydrogenation tandem reaction

Protic ionic liquids with nitrate anions were used as solvents and catalysts for a three-component oxidative dehydrogenation tandem reaction via the coupling and hydroarylation of benzaldehyde, aniline, and phenylacetylene to a quinoline derivative. The reaction was supported by air and microwave irradiation. The presence of nitrate as counter anion in the protic ionic liquids was essential for the reaction.

Influence of Ionic Liquids on an Iron(III) Catalyzed Three-Component Coupling/Hydroarylation/Dehydrogenation Tandem Reaction

A three-component oxidative dehydrogenation tandem reaction via the coupling and hydroarylation of benzaldehyde, aniline and phenylacetylene to a quinoline derivate was catalyzed by an iron-containing ionic liquid. The reaction was air mediated and could be performed under neat conditions. The iron(III) of the ionic liquid was the oxidizing species.

CuSO4–d-glucose, an inexpensive and eco-efficient catalytic system: direct access to diverse quinolines through modified Friedländer approach involving SNAr/reduction/annulation cascade in one pot

A one-pot, efficient approach to quinoline synthesis, directly from 2-bromoaromatic aldehydes/ketones in a H₂O–EtOH mixture involving a sequence of S_NAr/reduction/annulation cascade using CuSO₄-d-glucose, is devised.

Metal-free aerobic one-pot synthesis of substituted/annulated quinolines from alcohols via indirect Friedländer annulation

Metal-free one-pot aerobic synthesis of highly functionalized/annulated quinolines is devised in excellent yields from viable alcohols via indirect Friedländer annulation.

Synthesis of 1,4-dihydroquinoline derivatives under transition-metal-free conditions and their diverse applications

A transition-metal-free process for the synthesis of 1,4-dihydroquinoline derivatives starting from simple enaminones with aldehydes via intermolecular cascade cyclization in a one-pot protocol is developed. This methodology affords a variety of products in moderate to good yields. Particularly, the use of the enaminone fragment in 1,4-dihydroquinoline derivatives 3 as a leaving group for further diverse applications with C-nucleophiles is proved to be feasible.

Use of ionic liquids as neoteric solvents in the synthesis of fused heterocycles

Medicinal chemistry has been benefited by combinatorial chemistry and high-throughput parallel synthesis. Ionic liquids reduce the materials and energy intensity of chemical processes and products, minimize or eliminate the dispersion of harmful chemicals in the environment, maximize the use of renewable resources and extend the durability and recyclability of products. It is possible to tune the physical and chemical properties by varying the nature of the cations and anions. Ionic liquids can be easily recovered, cleaned up, and reused repeatedly.

Palladium catalyzed one-pot synthesis of 2-(pyridin-4-yl) quinolines via a multicomponent unprecedented reaction of pyridine-4-carbaldehyde, 2-iodoaniline and triethylamine

Palladium catalyzed synthesis of 2-(pyridin-4-yl) quinoline with an unprecedented participation of Et₃N was achieved in a novel multicomponent reaction of pyridine-4-carbaldehyde, 2-iodoaniline and triethylamine.

A facile one-pot Friedländer synthesis of quinoline derivatives

A facile one-pot Friedländer synthesis has been developed for quinolines using GdCl3.6H2O as the novel catalyst. The method is simple, efficient and rapid to afford the quinolines in very good yields.

Nickel-catalyzed cyclization of 2-iodoanilines with aroylalkynes: an efficient route for quinoline derivatives

An efficient and convenient nickel-catalyzed cyclization of 2-iodoanilines with alkynyl aryl ketones to give 2,4-disubstituted quinolines was developed. The reaction can be employed for the synthesis of naturally occurring quinoline derivatives in good yields. On the basis of the regiochemistry of the products, the possible pathway for the reaction is via the formation of o-aminochalcone.

A highly effective one-pot synthesis of quinolines from o-nitroarylcarbaldehydes

A highly effective one-pot Friedländer quinoline synthesis using inexpensive reagents has been developed. o-Nitroarylcarbaldehydes were reduced to o-aminoarylcarbaldehydes with iron in the presence of catalytic HCl (aq.) and subsequently condensed in situ with aldehydes or ketones to form mono- or di-substituted quinolines in high yields (66-100%).

Concerning the mechanism of the Friedländer quinoline synthesis

Detailed experiments regarding the mechanism of the Friedländer synthesis of quinolines from o-aminobenzaldehydes and simple aldehydes or ketones are described. Under the basic or acidic conditions commonly used in this reaction, it is concluded that the first step involves a slow intermolecular aldol condensation of the aldehyde or ketone with the o-aminobenzaldehyde. The aldol adduct 5 generated in this manner then undergoes very rapid cyclization to 4, which subsequently loses water to produce the quinoline derivative 8. Both 5 and 4 are too short lived to be detectable (TLC), even when deliberately generated by other means. It is also shown that E-enones corresponding to 6, i.e., the aldol dehydration product, are converted into quinolines (e.g., 21a and 21b from 17a and 17b) under basic or acidic conditions. Such enones are not detected as intermediates in the base-induced Friedländer synthesis, even though certain congeners (17b) would be easily observable. Under acidic conditions these enones are too short lived to be detectable. Schiff bases derived from 2-aminobenzaldehyde (18a) and aldehydes or ketones can be generated under special conditions, but they show reactivity patterns different from those seen in the usual Friedländer condensations. Thus, the ytterbium-triflate-catalyzed reaction of aldehydes with 18a at room temperature in toluene generates the E-Schiff bases (33, R1 = H), from which isomeric mixtures of tetrahydroquinoline derivatives 26 are formed exclusively. At higher temperatures, the E-Schiff bases 33 are isomerized to the Z-Schiff bases 34, from which the 3-substituted quinoline derivatives 24 are formed as the major products under appropriate conditions. Also, the ytterbium-triflate-catalyzed reaction of 18a with the pyrrolidine enamines of the methyl-n-alkylketones 38a,b produces mixtures in which the 2-monosubstituted kinetic products 37b,d predominate over the 2,3-disubstituted thermodynamic products 21c,e by a factor of 4:1 to 5:1. These results are opposite to those observed under the usual basic or acidic Friedländer reactions with methyl-n-alkylketones, where the thermodynamic products are usually strongly favored. The unusual kinetic:thermodynamic product ratios observed with 38a,b are ascribed to the generation and rapid cyclization of mixtures of the Schiff bases 35 and 36, in which the kinetic isomer 35 is highly predominant. Key words: mechanism, Friedländer synthesis, quinolines, intramolecular aldol reactions, Schiff bases, tetrahydroquinolines, high kinetic:thermodynamic product ratios.

An Efficient Synthesis of an αvβ3 Antagonist

A practical preparation of an alpha(v)beta(3) antagonist is reported. The antagonist consists of three key components, a tetrahydronaphthyridine moiety, a beta-alanine moiety, and a central imidazolidone moiety. The tetrahydronaphthyridine component was prepared using two different methods, both of which relied on variations of the Friedländer reaction to establish the desired regiochemistry. The beta-alanine component was prepared using Davies' asymmetric 1,4-addition methodology as the key stereo-defining step. The central imidazolidone portion was created from these two components using an effective three-step cyclization protocol. Thus, a highly convergent process for the drug candidate was defined.

Reaction of alpha-oxoketene-N,S-arylaminoacetals with Vilsmeier reagents: an efficient route to highly functionalized quinolines and their benzo/hetero-fused analogues

A simple, highly efficient, and regioselective synthesis of functionalized quinolines through Vilsmeier cyclization of a variety of alpha-oxoketene-N,S-anilinoacetals has been reported. The cyclization is found to be facile with N,S-acetals bearing strongly activating groups on aniline, whereas yields of quinolines are moderate in other cases. The reaction could also be extended for the synthesis of substituted tricyclic benzo[h]quinoline, pyrido[2,3-h]quinoline, 4,7-diphenylphenanthroline, and tetracyclic quino[8,7-h]quinoline by performing a Vilsmeier reaction on N,S-acetals derived from 1-naphthylamine, m-phenylenediamine, o-phenylenediamine, and 1,5-diaminonaphthalene, respectively. A few of the newly synthesized quinolines are subjected to further transformation to afford 2-unsubstituted (Raney-Ni/Ethanol), quinoline-5,8-quinone (NBS/H(2)SO(4)), or 2-alkyl/aryl aminoquinolines through sequential m-CPBA oxidation to the corresponding (2-methylsulfonyl)quinoline followed by replacement with appropriate amines. Similarly, cycloannulation of a few 2-methylthio-3-benzoylquinolines with hydrazine hydrate under microwave irradiation afforded the corresponding substituted and fused pyrazolo[3,4-b]quinolines in excellent yields, whereas TBTH/AIBN-mediated cyclization of the corresponding 3-(2-bromobenzoyl)-2-methylthioquinolines yielded the corresponding benzothiopyrano-fused quinolines through radical translocation.